Step 2.    We make use of the idea in [1]}
with modifications to derive the estimation of \(\phi _{y}\) .
Differentiating \((\ref {mass})\)  with respect to \(y\)  and then
multiplying the resulted equation by \(\phi _y/\rho ^3\)  to get
\(\begin{array}{ll}&\displaystyle (\frac{\phi _y^2}{2\rho ^3})_\tau +(\frac{u\phi _y^2}{2\rho ^3})_y+\frac{\psi _{yy}\phi _y}{\rho ^2}= -\frac{\phi _y}{\rho ^3}({\bar{u}}_{yy}\phi +{\bar{\rho }}_{yy}\psi +2{\bar{\rho }}_y\psi _y).\end{array}\) 
